In conventional wireless cellular networks, the initial rollout typically involves installation of macro base stations to provide wireless cellular coverage for mobile units. A macro base station comprises multiple transceiver units, outputs relatively high power (that is, 10 watts or more) to its antenna(s) and is communicatively coupled to a telephone network via a backhaul connection. The backhaul connection includes a T1 connection (in the United States) or an E1 connection (in Europe) to a base station controller (BSC) which is, in turn, connected to a mobile switching center (MSC), and external telephone network. Because macro base stations output high power, they can provide large areas of coverage.
The capacity of a macro base station can be expanded to a limited degree by the addition of transceivers and antennas to the macro base station. Additional macro base stations can also be added to the cellular network. However, these measures have limitations due to interference among macro base stations as a result of their large coverage areas and high output power.
A solution to this capacity problem has been to add micro or pico base stations to the cellular network. Like a macro base station, a micro base station comprises multiple transceiver units and is communicatively coupled to a telephone network via a backhaul connection to the BSC and MSC. However, compared to the output power of a macro base station, a micro base station outputs relatively lower power (that is, in the range of 1-2 watts) to its antenna(s). A conventional pico base station is also typically communicatively coupled to a telephone network via a backhaul connection, but comprises only a single transceiver unit and typically uses an Internet protocol (IP) backhaul connection in which voice signals are converted to IP packets. A conventional pico base station also outputs even lower power (that is, less than one watt) to its antenna. Pico base stations can be located indoors, such as in offices, shopping centers, convention centers, and airports. In addition to having lower output power levels, micro and pico base stations for Code Division Multiple Access (CDMA) and broadband wireless protocols also support lower capacity levels than macro base stations due to their reduced processing power.
A drawback to this approach for adding capacity to the network is that the micro or pico base stations are located at sites where the additional capacity is needed and therefore require additional infrastructure for each site. Furthermore, they are not easily accessible for maintenance or upgrades. Also, because an additional backhaul link is required for each micro or pico base station, the backhaul links tend to increase installation and maintenance expense. Moreover, the coverage provided by the pico base stations is typically limited and often problematic in indoor deployments due to walls and building configuration.
Another issue with covering a large area with pico cells is that capacity demand is often dynamic with respect to location and loading. As users move about an area the capacity demands will shift to different locations. Network designers must often provision excess capacity, which can cause many pico cell resources to go underutilized. Also, for broader band technologies such as Universal Mobile Telecommunications System (UMTS), Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE) technologies, scattering multiple pico cells with lower output power and capacity to cover larger areas is inefficient due to the co-channel interference created by neighboring cells. Trunking gain can be achieved by distributing a higher level of capacity over the entire coverage area rather than individually deploying slices of the capacity at various points in the entire coverage area.